The present invention relates to a lithium secondary battery.
From the point of ionic conductivity, liquid electrolytes have hitherto been used as electrolytes constituting electrochemical devices such as batteries, capacitors, sensors, etc. However, there are problems such as causing damages of devices due to leakage of the liquid electrolyte.
On the other hand, recently, there are proposed secondary batteries in which solid electrolytes such as inorganic crystalline materials, inorganic glass, organic polymers, etc. are used as electrolytes. When these solid electrolytes are used, leakage of carbonate solvents does not occur and ignition of electrolytes can be diminished, and, hence, reliability and safety of the devices are improved as compared with when conventional liquid electrolytes containing carbonate solvents are used.
Furthermore, organic polymers are generally superior in processability and moldability, and the resulting electrolytes have flexibility and bending processability. As a result, design freedom becomes higher for the devices to which they are applied. Thus, development of them is expected.
However, at present, the organic polymers are inferior to other materials in ionic conductivity. For example, it has been widely attempted to use polyethylene oxide containing a specific alkali metal ion as polymer electrolytes (e.g., JP-A-2002-158039), but a value of ionic conductivity (1 mS/cm or higher at room temperature) which is practically required for electrolytes of lithium ion secondary batteries has not yet been obtained.
Furthermore, JP-A-2001-72877 discloses copolymers of boron-containing polymerizable monomers and other polymerizable monomers. The polymers are ion conductive materials which can be improved in transport number of charge carriers, and there are disclosed polymer electrolytes and electrochemical devices using the polymers.
However, the above-mentioned organic polymers are inferior to other materials in ionic conductivity. For example, the above publications disclose polymer electrolytes comprising polyethylene oxide containing a specific alkali metal salt, but these polymer electrolytes do not have a value of ionic conductivity (1 mS/cm or higher at room temperature) which is practically required for electrolytes of lithium secondary batteries. The object of the present invention is to provide a lithium secondary battery excellent in output characteristics in which a polymer electrolyte of high ionic conductivity is used.